Improve mypy's behavior for implicit returns.

See python/mypy#7511 though it seems this isn't fully
fixed, as it still warns for returns with no explicit
return None...

Author: Julian

pyproject.toml

@@ -50,4 +50,5 @@ multi_line_output = 3
 
 [tool.mypy]
 strict = true
+warn_no_return = false
 exclude = [".*/tests/.*"]